Rhizobacteria have long been known for their role in promotion of plant growth and biological control of plant pathogenic microorganisms. It is believed that plant growth-promoting rhizobacteria act by displacing or antagonizing plant pathogenic microorganisms. Many of these rhizobacteria produce antibiotics, the production of which is strongly correlated with inhibition of various pathogenic microorganisms in vitro and disease suppression in vivo.
Various microorganisms in soil, including Enterobacter aerogenes, Pseudomonas fluorescens, Pseudomonas cepacia, and Bacillus species, have been reported to produce antibiotics and antifungal compounds that effectively control or suppress phytopathogenic fungi both in vitro and in vivo. Bacteria of the genus Bacillus produce a variety of peptide antibiotics that are antibacterial and/or antifungal. Several Bacillus species, including B. subtilis, B. pumilus and B. cereus, have been shown to be antagonistic to plant pathogenic fungi and bacteria. For example, phytopathogen-antagonistic strains of B. subtilis have been reported to produce two peptide antibiotics: bacilysin, a dipeptide that inhibits yeast and bacteria; and fengycin (fengymycin), a lipopeptide antagonistic against phytopathogenic fungi such as Rhizoctonia solani. (Loeffler et al. (1986), J. Phytopathology 115: 204-213; Vanittanakom et al. (1986), J. Antibiotics 39: 888-901). Bacillus cereus produces mycocerein, another antifungal peptide (Wakayama et al. (1984), Antimicrobial Agents and Chemotherapy 26: 939-940). Additionally, biological control of Eutypa lata on grapevine by an antagonistic strain of Bacillus subtilis has been reported (Ferreira et al. (1991), J. Phytopathology 81: 283-287) In that instance, inhibition of mycelial growth and ascospore germination was correlated with the presence of an antibiotic substance in an ethanol extract of the B. subtilis strain.
The Bacillus genus is divided into three groups, for purposes of identifying and classifying Bacillus species (Claus & Berkeley (1986) "Genus Bacillus Cohn 1872 174.sup.AL ", pp. 1104-1139 in Bergey's Manual of Systematic Bacteriology, Volume II (Sneath, Mair, Sharpe and Holt, eds.), Williams & Wilkins Co., Baltimore). Most Bacillus species reported to be useful as biological control agents belong to the "subtilis" group; which includes B. subtilis, B. cereus, B. pumilus and B. licheniformis, among others. Of this group, Bacillus licheniformis is industrially valuable for its ability to produce secreted products, such as thermally stable alpha amylases (See, e.g., Saito (1973), Arch. of Biochem. & Biophys. 155: 290-298; Yuuki et al. (1985), J. Biochem. 98: 1147-1156). Moreover, Bacillus licheniformis is known for its ability to survive and proliferate in extreme environments, including high temperature and anaerobic conditions (B. licheniformis is a facultative anaerobe). Bacillus licheniformis is widely known to produce antibiotics such as bacillomycin, bacitracin, licheniformin and proticin (See Katz & Demain (1977), Bacteriol. Rev. 41: 449-474). However, the ability of this species to produce antifungal compounds, as well as its usefulness as a biological agent for the control of plant diseases, has remained largely unexplored (See Loeffler et al. 1986, supra.).
Because Bacillus licheniformis can grow anaerobically and at elevated temperatures, it is capable of inhabiting a wide variety of rhizospheres, including waterlogged soils and/or soils exposed to extremes of heat, such as in semi-arid or arid regions, or tropical and sub-tropical areas. For this reason, it would be advantageous to identify strains of Bacillus licheniformis that are antagonistic to plant pathogenic fungi and that produce antifungal compounds. It would be a further advantage to identify such Bacillus licheniformis strains having an antagonistic effect on a wide variety of phytopathogenic fungi. Such organisms could be used for biological control of fungal plant pathogens on a broad range of plant species, against a large variety of pathogens. Insofar as is known, Bacillus licheniformis strains having such characteristics have heretofore not been identified.